(1) Field of the Invention
The present invention relates to automatic machines for displacing an element, such as a shaping tool, over a work area in a predetermined manner. In particular, but not exclusively, the invention concerns an automatic machine for controlling the movement of a shaping tool which, during operation on a workpiece, is subjected to substantial reaction forces in the plane of movement of the tool across the workpiece.
(2) Description of the Prior Art
The economic advantage of being able to automatically carry out predetermined operations on a workpiece is well appreciated.
Known automatic machines include robot manipulator arms carrying a tool at their free ends. These robot arms generally comprise a number of articulated limbs whose relative orientations are controlled by respective servo control systems in response to `demand` signals read off a program tape. The actual sensing and displacement of the tool held at the free end of the arm is determined by a summation of the sensed relative positions of the limbs and a summation of their relative displacements. Such automatic machines are generally unsuitable for operations where substantial reaction forces are experienced since the inertia of the limbs and their flexions under the action of the reaction forces makes accurate absolute position sensing of the tool and its accurate movement very difficult. The use with certain robot arms of imaging systems to detect tool position does not solve the problem since image processing is slow and effectively introduces inertia into the servo control systems.
Other automatic shaping machines are known in which the effect of varying cutting reaction forces is compensated for by mounting the shaping tool on a massive carriage displaceable over the workpiece. This approach requires very large acceleration forces to be generated to achieve an adequate operating speed with the result that its mechanical implementation is large and complex.
British Pat. No. 1,438,414 (IBM) teaches an interesting approach to the problem of accurately moving a tool over a workpiece. In the system there described, coarse positioning is effected by moving the worktable carrying the workpiece while fine positioning is achieved by movement of the tool (an electron beam). Coarse and fine positioning are carried out in succession so that workpiece processing can only be carried out continuously over the small area accessible by fine position adjustment of the tool. Continuous operation over the range of workpiece locations achievable using the movable worktable is not possible.
The aforementioned automatic machines are generally numerically controlled, the desired movement path of the tool being defined by a succession of points with interpolation therebetween. The processing of the `demand` and feedback data of the servo control systems of the machines requires a considerable data handling capability resulting in expensive computer control systems, especially to interpolate between the main points at high data rates.
It is an object of the present invention to provide an automatic machine for displacing an element over a work area which is capable of compensating for varying forces exerted on said element and yet can effect relatively accurate and rapid movement of the element. In particular, the invention seeks to provide an improved automatic shaping machine such as a milling or routing machine.